1. Field of the Invention
The present invention generally relates to a motor and, more particularly, to a motor with an axial air gap.
2. Description of the Related Art
Referring to FIG. 1, a motor 8 with an axial air gap is disclosed by Taiwan Patent Publication No. 201018371 entitled “THIN DISSIPATING FAN”. The motor 8 includes a base 81, an impeller 82 and a magnet plate 83. The base 81 is mounted with a coil unit 811. The impeller 82 is rotatably coupled with the base 81. The magnet plate 83 is rotatably coupled with a bottom face of the impeller 82, with an axial air gap formed between the magnet plate 83 and the coil unit 811.
When the coil unit 811 of the motor 8 is electrified to generate alternating magnetic fields, the alternating magnetic fields will drive the magnet plate 83. Magnetic lines of the magnet plate 83 have a magnetic flux linkage with the coil unit 811 through the axial air gap, thereby driving the impeller 82 to rotate. However, when viewing from the top of the motor 8, the magnet plate 83 extends way over the coil unit 811 in a radial direction. As a result, inner and outer circumferential faces of the magnet plate 83 are far away from the coil unit 811. As an undesired effect, magnetic lines emitted from the parts of the magnet plate 83 around the inner and outer circumferential faces of the magnet plate 83 may not be able to reach the coil unit 811 for a magnetic flux linkage. Those magnetic lines not contributing to the magnetic flux linking effect are called leaked magnetic fluxes, which interact with the coil unit 811 to form a high frequency resonance effect that leads to generation of noises. Therefore, the operation of the motor 8 is interfered.
Referring to FIG. 2, another motor 9 with an axial air gap is disclosed by Taiwan Publication No. 201023481 entitled “ROTOR OF A MOTOR”. The motor 9 includes a rotor 91 having a permanent magnet 911. The rotor 91 is rotatably coupled with a base 92 mounted with a stator coil 93. An axial air gap exists between the stator coil 93 and the permanent magnet 911. In such an arrangement, the stator coil 93 can drive the rotor 91 to rotate via the permanent magnet 911.
When viewing from the top of the motor 9, the permanent magnet 911 is located within an axial projection range of the stator coil 93. Namely, the parts of the permanent magnet 911 around inner and outer circumferential faces of the permanent magnet 911 are covered by the axial projection range of the stator coil 93. This prevents magnetic flux leakage from happening. As such, the motor 9 does not generate much noise, and the operation thereof is not interfered. However, the ratio of radial length between the stator coil 93 and the permanent magnet 911 is not properly set, resulting in an excessive amount of material consumption of the stator coil 93 or the permanent magnet 911 and failing to meet economic needs.